Two astronauts began Sunday a second of four scheduled space walks of the Endeavour shuttle's final mission to the International Space Station (ISS), the US space agency announced.
Drew Feustel and Mike Fincke emerged from a decompression chamber at 0605 GMT, or 11 minutes ahead of schedule, starting their six-and-a-half-hour excursion.
Feustel and Fincke will refill the station's radiators with ammonia, said officials of the National Aeronautics and Space Administration.
They also will complete venting the early ammonia system, lubricate a left-side solar joint and parts of Dextre, a two-armed space station robot capable of handling delicate assembly tasks currently performed by spacewalkers.
Endeavour Mission Specialist Greg Chamitoff will be inside the International Space Station, coordinating communications between the spacewalkers and mission control in Houston and choreographing the spacewalkers' activities, accoring to the NASA officials.
Greg Johnson and Expedition 27 crew member Cady Coleman will be operating the space shuttle and space station robotic arms in the later portion of the spacewalk.
This is the 246th spacewalk conducted by US astronauts, the 157th in support of space station assembly and maintenance, the fifth for Feustel and the seventh for Fincke.
Endeavour blasted off on its final mission on Monday with six astronauts on board - five Americans and one Italian - and docked at the ISS on Wednesday.
The Endeavour mission is being commanded by astronaut Mark Kelly, the husband of US Representative Gabrielle Giffords, who is recovering after being shot in the head at a January political meeting with local voters.
The shuttle will remain at the space station until May 30, returning to the United States on June 1.
The 30-year US space shuttle program formally ends later this year with the flight of Atlantis, leaving Russia's space capsules as the sole option for world astronauts heading to and from the orbiting research lab.